Therac-25 was a new generation medical linear accelerator for treating cancer. It incorporated the most recent computer control equipment. Therac-25’s computerization made the laborious process of machine setup much easier for operators, and thus allowed them to spend minimal time in setting up the equipment. In addition to making setup easier, the computer also monitored the machine for safety. With the advent of computer control, hardware based safety mechanisms were transferred to the software. Hospitals were told that the Therac-25 medical linear accelerator had "so many safety mechanisms" that it was "virtually impossible" to overdose a patient.
Normally, when a patient is scheduled to have radiation therapy for cancer, he or she is scheduled for several sessions over a few weeks and told to expect some minor skin discomfort from the treatment. The discomfort is described as being like a mild sunburn over the treated area. But in this case on safety critical software, you will find that some patients received much more radiation than prescribed.
Therac-25
A computer controlled medical linear accelerator for treating cancer
Therac-25 was a new generation machine that incorporated the most recent computer control equipment. The machine targeted electron or X-ray beams on cancerous tissue to destroy it. Electron beams were used to treat shallow tissue, while X-ray beams could penetrate with minimal damage to treat deep tissue.
When a doctor decides that a patient needs radiation therapy, that patient is given a prescription that indicates to the medical linear accelerator operator how many rads (radiation absorbed dose) the patient should receive over how many total treatments. In addition, the prescription indicates the location where the radiation should be applied. The patient schedules a time (or times) to receive treatment.
Standard procedures then determine whether, on any particular appointment, the operator is to set up the equipment for electron or X-ray beam treatment. The patient is asked to lie in the appropriate position on the treatment table and the table is rotated to place the diseased part of the patients' body in the path of, and at the appropriate distance from, the radiation beam. The operator then does whatever mechanical setup is required and leaves the room to program the treatment data into the machine. After doing this, the operator presses the button that actuates the treatment routine. The patient is then helped off the treatment table and ushered out. After the appropriate forms have been filled out, the next patient is admitted.
Therac-25’s computerization made this laborious process much easier for operators, and allowed them to spend minimal time in setting up the equipment. Operators were thus freed to spend more time talking with and helping the patient.
In addition to making setup easier, the computer also monitored the machine for safety. Previous machines had safety devices as a part of the hardware of the machine. Among other things, these safety devices kept the machine from delivering doses of radiation that were too high. So, with the advent of computer control, these hardware based safety mechanisms were transferred to the software. Hospitals were told that the Therac-25 medical linear accelerator had "so many safety mechanisms" that it was "virtually impossible" to overdose a patient.
How Radiation Therapy Works
What Radiation Therapy Is
Radiation therapy for cancer is the exposure of cancerous tissue to ionizing radiation. This is usually done by what is called "external" therapy, using electron, X-rays or gamma rays to treat the tissue. This therapy may occur either before or after surgery, or in the place of surgery.
Therac-25 was a 3rd generation radiation therapy machine for external radiation therapy. It used either electron beam or X-rays to treat tissue.
Why Radiation Therapy Works
Cancer cells usually multiply faster than most other cells in the body. Tissue composed of these quickly-dividing cells can be shrunken by disabling its genetic material. By doing this, ionizing radiation interferes with the cancerous tissue’s ability to grow.
Unfortunately, the radiation makes no distinction between cancerous cells and other rapidly dividing body tissues. Skin and hair are some of the most noticeably hurt tissues after treatment, and treatment may produce skin lesions and hair loss. These tissues have cells that rapidly divide and the radiation halts their development. But they are usually able to recover from this assault and return to normalcy. Nevertheless, skin lesions and hair loss are not an unusual side effect of radiation therapy.
What a Treatment Session is Like
Radiation therapy is usually done in a series of sessions occurring over several weeks, allowing the effect of the radiation to build up over time. The treating doctor will determine the specific number of treatments, the dosage at each treatment, and the schedule. During treatment, the doctor will usually see the patient once a week to check on general health, side effects, and the progress of the treatment.
Before the series of treatments occurs, a radiation therapy technician will outline the specific area to be treated with a marking pen, indelible ink or silver nitrate.
Depending on the body area to be treated, the patient would need to remove his or her clothing and put on a hospital gown. After going to the radiation therapy room, they would then either lie on a treatment table or sit in a special chair (Therac-25 had a table). The marks on the skin are used to guide the machine operator in locating the precise area to be treated. Once the machine is sset up, the operator leaves the room for a control room nearby. This protects the operator from prolonged exposure to low-level radiation that might scatter from the machine (an operator may treat as many as 30 patients in a day). From there, the operator will turn on the treatment machine while he/she watches. With the Therac-25, this was accomplished by means of a television camera and monitor. During radiation therapy, the treatment machine makes a buzzing noise. Treatments are typically brief and painless, normally lasting 1 to 5 minutes. Total time in the treatment room will usually be 5 to 15 minutes.
Table of Contents
Normally, when a patient is scheduled to have radiation therapy for cancer, he or she is scheduled for several sessions over a few weeks and told to expect some minor skin discomfort from the treatment. The discomfort is described as being like a mild sunburn over the treated area. But in this case on safety critical software, you will find that some patients received much more radiation than prescribed.
Therac-25
A computer controlled medical linear accelerator for treating cancerTherac-25 was a new generation machine that incorporated the most recent computer control equipment. The machine targeted electron or X-ray beams on cancerous tissue to destroy it. Electron beams were used to treat shallow tissue, while X-ray beams could penetrate with minimal damage to treat deep tissue.
When a doctor decides that a patient needs radiation therapy, that patient is given a prescription that indicates to the medical linear accelerator operator how many rads (radiation absorbed dose) the patient should receive over how many total treatments. In addition, the prescription indicates the location where the radiation should be applied. The patient schedules a time (or times) to receive treatment.
Standard procedures then determine whether, on any particular appointment, the operator is to set up the equipment for electron or X-ray beam treatment. The patient is asked to lie in the appropriate position on the treatment table and the table is rotated to place the diseased part of the patients' body in the path of, and at the appropriate distance from, the radiation beam. The operator then does whatever mechanical setup is required and leaves the room to program the treatment data into the machine. After doing this, the operator presses the button that actuates the treatment routine. The patient is then helped off the treatment table and ushered out. After the appropriate forms have been filled out, the next patient is admitted.
Therac-25’s computerization made this laborious process much easier for operators, and allowed them to spend minimal time in setting up the equipment. Operators were thus freed to spend more time talking with and helping the patient.
In addition to making setup easier, the computer also monitored the machine for safety. Previous machines had safety devices as a part of the hardware of the machine. Among other things, these safety devices kept the machine from delivering doses of radiation that were too high. So, with the advent of computer control, these hardware based safety mechanisms were transferred to the software. Hospitals were told that the Therac-25 medical linear accelerator had "so many safety mechanisms" that it was "virtually impossible" to overdose a patient.
How Radiation Therapy Works
What Radiation Therapy Is
Radiation therapy for cancer is the exposure of cancerous tissue to ionizing radiation. This is usually done by what is called "external" therapy, using electron, X-rays or gamma rays to treat the tissue. This therapy may occur either before or after surgery, or in the place of surgery.Therac-25 was a 3rd generation radiation therapy machine for external radiation therapy. It used either electron beam or X-rays to treat tissue.
Why Radiation Therapy Works
Cancer cells usually multiply faster than most other cells in the body. Tissue composed of these quickly-dividing cells can be shrunken by disabling its genetic material. By doing this, ionizing radiation interferes with the cancerous tissue’s ability to grow.Unfortunately, the radiation makes no distinction between cancerous cells and other rapidly dividing body tissues. Skin and hair are some of the most noticeably hurt tissues after treatment, and treatment may produce skin lesions and hair loss. These tissues have cells that rapidly divide and the radiation halts their development. But they are usually able to recover from this assault and return to normalcy. Nevertheless, skin lesions and hair loss are not an unusual side effect of radiation therapy.
What a Treatment Session is Like
Radiation therapy is usually done in a series of sessions occurring over several weeks, allowing the effect of the radiation to build up over time. The treating doctor will determine the specific number of treatments, the dosage at each treatment, and the schedule. During treatment, the doctor will usually see the patient once a week to check on general health, side effects, and the progress of the treatment.Before the series of treatments occurs, a radiation therapy technician will outline the specific area to be treated with a marking pen, indelible ink or silver nitrate.
Depending on the body area to be treated, the patient would need to remove his or her clothing and put on a hospital gown. After going to the radiation therapy room, they would then either lie on a treatment table or sit in a special chair (Therac-25 had a table). The marks on the skin are used to guide the machine operator in locating the precise area to be treated. Once the machine is sset up, the operator leaves the room for a control room nearby. This protects the operator from prolonged exposure to low-level radiation that might scatter from the machine (an operator may treat as many as 30 patients in a day). From there, the operator will turn on the treatment machine while he/she watches. With the Therac-25, this was accomplished by means of a television camera and monitor. During radiation therapy, the treatment machine makes a buzzing noise. Treatments are typically brief and painless, normally lasting 1 to 5 minutes. Total time in the treatment room will usually be 5 to 15 minutes.
GO TO: 5. HISTORY OF THERAC-25